This invention relates to IC manufacture and assembly, and more particularly, to a method for mounting solder balls on a semiconductor substrate package.
Many different constructions and package lead counts are used by the semiconductor industry to protect and connect IC devices. One of the most current package formats is called a Ball Grid Array (BGA). This package consists of a ceramic or laminated PC board substrate onto which have been printed, plated or deposited a pattern of electrically conductive traces. A semiconductor die is mounted on one side of the substrate and electrically connected to these conductive traces. The conductive traces then branch out into an array pattern on the substrate surface. The array pattern can be on the same side of the substrate where the semiconductor die is mounted, or on the opposite side. Solder balls, which can be any one of a number of low temperature, conductive alloys, are then reflow soldered to the end of the conductive traces or "pads" in the array pattern. The solder balls provide the means for electrically connecting the conductive traces to the system in which it functions.
Several methods are currently used to mount the solder balls onto the BGA package substrate. In one such method, soldering flux is applied to the package pads, after which a fixturing device or pickup and placement head is used to place the preformed solder balls, individually or en mass, onto the pads. The package is then heated to the melting point of the solder alloy, which then wets and bonds to the package pads.
An alternate method uses a printing or dispensing fixture or process to apply measured quantities of solder paste, a mixture of fine solder particles in a paste flux vehicle, onto the package contact pads. The paste can be melted by itself to form the ball contacts, or, by placing preformed solder balls into the solder paste, used to attach the preformed solder balls to the package pads.
These soldering methods, and others, require a "flux" to remove contamination and oxides from the surfaces of the package pads and the solder balls or paste particles, to achieve a satisfactory wetting of the surface by the molten solder. However, this flux remains on the package after the solder is melted, necessitating a subsequent cleaning step. This additional step adds cost and manufacturing time to the construction of this type of semiconductor package. Currently, there are "no clean" fluxes available which become inert after soldering, but even those leave an undesirable residue on the surface.
An inert gas atmosphere such as nitrogen (N.sub.2) could be used to surround the soldering process if the package substrate pads and the solder ball preforms were perfectly clean and oxide free. This would eliminate the need for flux and the resultant flux residue problem, but nitrogen will not remove contaminants, just inhibit them from forming at elevated temperatures. The flux, which is tacky, performs another very useful function, namely holding the solder ball preforms in position on the substrate pads prior to the soldering operation.
The present invention addresses an improved method of installing and soldering the solder balls onto the package substrate which eliminates the need for soldering flux and the attendant disadvantages.